1. Field of the Invention
The present invention generally relates to methods and apparatus for separating water borne residuals created during the high pressure waterjet cutting of material in nuclear reactors and more particularly to a rapid process for continuously and rapidly removing same when using particulate matter in the Waterjet cutting such as garnet.
2. Description of the Prior Art
The waterjet cutting process when used in nuclear reactors uses an approximately 50,000 psi water stream which includes a small amount of particulate shot material such as garnet pellets to assist in the waterjet cut through the stainless steel or any other metal encountered in the reactor. Garnet is a form of silicate with traces of iron, aluminum as well as other possible trace elements such as chrome, magnesium, calcium, manganese and titanium. The typical chemical formula for garnet is: A3B2(SiO4)3, where A=iron, manganese, calcium or magnesium and B represents elements such as aluminum, iron, chromium or titanium. The waterjet garnet is substantially fractured by the high-pressure impact of the garnet on the cut stainless steel of the reactor. This results in a variety of particle sizes from less than 1 micron to about 300 microns.
Due to the high energy fracturing of the garnet during the waterjet cutting the creation of very small colloidal particles of garnet was seen. This is true for other particles used in Waterjet cutting such as stainless steel and others, although the amount of such created colloids may change. During this process Si(OH)4 is created which, due to its chemistry (and the approximate neutral carrying water) the presence of charged hydroxyl complexes such as: (HO)3 SiO are allowed. These bear a negative charge. This negative charge is a universal problem when particle removal from water is desired. Further, the creation of other typical very small colloidal particles of less than 10 microns that may have a variety of SiOx structures will also bear a negative net charge. Colloids are any particles, by definition, between about 0.005 and 10 microns in size, regardless of their chemical or biological composition.
A universal property of water borne colloidal silica, in neutral to near neutral pHs, is that they will bear a negative charge. This negative charge, combined with the small size creates a substantial particle removal problem.
Although colloidal particles are small, they have a very large surface area which permits the colloid to scatter light far beyond what might be suggested by its mass. The surface area value for the xe2x80x9ctypical particlexe2x80x9d is in the range of 250-350 square meters per gram. This results in a very, very low quantity of colloidal silica causing significant turbidity (light scatter and failure to pass light) in any colloidally contaminated water. It has been found that the waterjet process water even when filtered through a 1 micro filter will still absorb over 98% of 400 nano-meter light at a distance of 5 feet in depth.
Another feature of the small mass of each colloid particle is that its electric charge (negative) to its mass is quite high. This high charge allows colloidal particles to repel other particles vigorously and, hence, hinders efficient removal or filtration or centrifuging (hydro cyclones, cyclone separators and centrifuges).
The negative charge is measured in a known manner using a Zeta potential meter. Typical waterjet cutting polluted reactor water was tested to have a value of between minus 20 and minus 40. An acceptable value for treated and filtered water is minus 5 to plus five as published by the Zeta meter manufacturer.
When xe2x80x9cun-charge neutralizedxe2x80x9d colloidal particles are trapped in or on a filtering surface, the repulsion energy grows quickly. The net result is that the filtering surface is quickly xe2x80x9cpluggedxe2x80x9d with a very low mass, but which mass also has a high repulsion energy to the acceptance of any more negatively charged mass in the water. Also, some colloidal particles, which remain uncharged or are neutralized, will not be filtered out at all, even with a 0.5 micro filter, and will pass into the filtrate causing turbidity.
When untreated colloidal particle filtration continues for a brief time, the bulk of the suspended mass in the water, which is non charged, cannot be efficiently filtered from the water due to premature filter plugging by the negatively charged colloidal particles. Experiments showed that process water that was even first passed through a cyclone separation continued to rapidly plug down field 3M filters of both 3 micron and 1 micron. This is due to the retained colloids and negative Zeta potential of the water.
The problem is further exasperated by the fact that the spent cutting water from the reactor also contains metal fines or what is referred to as swarf. None of these particles are colloidal. But, a very small amount is fractured by the waterjet process into a colloidal state. This occurs because about 0% of the suspended_stainless swarf, by weight, is fractured so vigorously into small particles that the chromium and nickel from the steel is forced into solution. Colloidal particles of metal are likely to be in a hydrated form similar to the hydrated/ionized silica and hence would contribute to the negative repulsion of the spent water stream.
This generation of fractured garnet and metal particles, during waterjet cutting in nuclear reactors that are less than 1 micron is size creates a huge problem in that prior art normal filtration is seriously hindered. In order to remove these very small particles, a very fine filter is required even though the overwhelming mass of the residuals are large enough to be trapped by a relatively high capacity corrugated depth filter such as a 2-10 micro fabric or a paper filter.
The very fine particles require fine filters, which by definition have a far lower capacity than a crasser filter. The use of fine filters leads to a very high generation of filter body waste, all of which will also be highly radioactive and thus extremely costly to dispose of.
The very fine, colloidal, particles further retard typical filtration because they are very negatively charged, repelling each other, leading to even a faster decline in filtering capacity and yet producing an even higher quantity of xe2x80x9cdeadxe2x80x9d filters. This phenomena was confirmed through testing. During testing, fine filters were clogged in less than ten minutes with a fraction of the solids loading normally observed. Thus it was seen that the particle distribution for the whole body of total suspended waterjet particles in the process water, being from less than 1 micron to about 300 microns, is in a range that is totally unsuitable for mechanical separation with any level of efficiency.
It should be noted that during our testing, untreated process water was subjected to both a Krebs hydro cyclone and a Lykos liquid solids separator. Neither method could provide a removal efficiency, on average, of even 50%. This was due to the small negatively charged particles and the large negative Zeta potential of the water. Treatment of the waste water prior to mechanical separator treatment indicated some potential in substantially improving this mechanical method, but with treatment costs included, other methods of residual removal indicated much less expensive potential.
All the mentioned treatment systems were slow and inefficient and totally inappropriate to meet the needs for rapid removal of the pollutants within less than a minute and preferably within seconds.
In order to rapidly remove the waterjet residuals from the process water within the mentioned time factors, some form of traditional pre-filtration chemical treatment seemed to be required.
Properly treated nuclear reactor water needs a treatment and collection system, which will rapidly (within seconds) achieve the following required points:
a bulk solids in water separation efficiency of 98%+
the addition of any treatment solids requires minimization
high radioactivity due to the metal swarf needs a robust treatment system that will function under water
the process treatment system has to function at 1,000 gallons per minute.
the space available for treatment equipment is very restricted.
To achieve these goals mechanical separation techniques were evaluated but were found completely unsatisfactory.
The known mechanical technology used to remove turbidity and color from waters of all kinds has been established in the United States since early in the twentieth century. While there are a variety of approaches they all involve some variation of the following steps:
Coagulation
Flocculation
Sedimentation or Settling
Final Filtering.
These techniques are defined as follows:
Coagulation is the process of removing the negative, repulsive charge on the smallest particles that create the turbidity and filtering problems of premature filter clogging. Classically, this is done with either +3 charged aluminum or plus 3 charged iron salts. In more recent times, in circumstance the will allow for the extra cost, or where low turbidity exits, organic cationic polymers are used to reduce the negative charge (Zeta potential) of the water. Once the charge has been removed, the very small particles can begin to agglomerate into larger particles. This aids direct filtration as well as settling.
Flocculation aids the pace at which the very small charge neutralized particles will clump into bigger particles, large organic molecules called flocculants are used. There a great many of them and some work better than others on any particular water. These agents bridge between particles aiding in more rapidly getting them into filterable, settleable sizes.
Settling is defined as the time it takes to adequately coagulate and flocculate water in order for it to self-settle into clear treated decantable water is hours, days or even weeks. However, in cases where there is not sufficient room to store large quantities of treated water, awaiting settling to clarity, filtration is used after a period of reaction between the coagulants, flocculants and the waste water. Depending upon the level of turbidity, this period is widely considered to, be between xcx9c15 minutes and several hours.
Water that has been treated by the previous steps can be readily filtered. This is typically done using rotary vacuum filters with blade scrappers or plate and frame pressure filters. Very slightly turbid water may, in some instances, simply be treated using sand and granular activated carbon (GAC) back-washable filters. Highly turbid waters are seldom if ever final filtered in this way.
Because of the reasons presented above, simple, direct filtration of highly charged water, such as the waterjet water, does not work from a practical point of view. Experience at the Framatome Mill Ridge testing facility during 1999 proved this point. Even filtration using some fine filter precoating did not work. These results are anticipated by the nature of the waterjet process and the resultant water quality, which by American Water Works Association falls into the highest class of 3 classes of turbid water.
Certain U.S. Patents teach processes for treating reactor water. These patents are as follows:
U.S. Pat. No. 6,156,194 teaches a method of treating reactor water that is contaminated with both radioactive metal cutting fines that come from the reactor and non-radioactive waterjet cutting particles that come from the waterjet cutting process.
The separation method taught is to magnetically filter the magnetic metal cuttings from the non-magnetic garnet waterjet particles to thus separate the two and subject the two streams to filtration that can produce recyclable reactor water.
This patent does not address the problems posed by colloidal particles produced by the garnet particle waterjet process and there is no indication therein that this process can rapidly remove these particles within seconds.
U.S. Pat. No. 5,637,029 teaches a method of recovering desired size abrasive shot material from a slurry recovered from a liquidxe2x80x94abrasive blast cleaner apparatus. The slurry is filtered for different size shot to recover usable materials.
Again, this patent does not address the problems posed by colloidal particles produced by the garnet particle waterjet process and there is no indication therein that this process can rapidly remove these particles within seconds.
A careful review of both of the above patents shows that it is known to remove the metal cuttings and garnet from the water in a reactor in which a waterjet cutting operation using garnet was used. However, the known removal method uses magnetic filters to separate the metal cuttings from the garnet particles. A filtration of the water slurry for collecting desired size particles is also shown. However, none of these patents recognize the problem of rapidly treating, within seconds, colloidal, negatively charged particles of both particles such as garnet and metal cuttings, swarf, to provide clean turbidity free recyclable reactor water, Also, the processes described therein do not teach any systems that will optimally remove the negative charge on the colloidal suspensions nor any optimized chemicals and their composition to flocculate them into larger particles making a special mechanical filtration feasible.
Thus a rapid process for processing water from a nuclear reactor contaminated by waterjet particles such as garnet particles and metal cuttings formed as colloidal suspensions of metal cutting particles and swarf to an acceptably clean level within seconds was sorely needed.
The present invention solves the problems associated with prior art processes and others by providing a rapid method and apparatus for cleaning very small colloidal material from the abrasive material used in waterjet cutting (such as gamet) and metal particles from a cut reactor section. Both are found in the reactor water after a high-energy waterjet cutting of metal such as stainless steel using abrasive shot cutting particles such as garnet particles. All abrasive shot materials will yield some colloidal suspension. The high energy of the waterjet cutting process causes fracturing of the abrasive material as well as the cut metal fines producing a negatively charged colloidal suspension having a large area. This colloidal suspension causes water turbidity and makes normal filtration techniques impractical.
To clean the reactor water effectively and rapidly, a process was optimized to draw turbid water from a reactor, which had been subjected to garnet waterjet cutting. This water was first subjected to a Zeta treatment followed by the addition of coagulants and flocculants. The treated water was then sent to a filtering tank to which a precoat was added and which separates the solids from the clean liquid and sends the liquid to a secondary filtration station from which clean water which is Zeta adjusted and which has all colloidals removed may be recycled to the reactor.
In view of the foregoing it will be seen that one aspect of the present invention is to provide a method of very rapidly treating reactor water having waterjet cutting colloidal particles.
Another aspect is to provide a method of removing colloidal cutting particles such as garnet and metal cutting swarf from reactor water.
Yet another aspect is to provide an optimized reactor water treatment process using specific filtration and filter precoat techniques.
Still yet another aspect is to provide an optimized reactor water treatment process using specific negative charge removal and flocculants techniques.
Still yet another aspect of the present invention is to provide a method of treating reactor water having waterjet cutting colloidal particles within seconds.